Hand lay-up is the oldest and simplest method for producing glass reinforced structures from polyester resins and is also very frequently used for making the mould itself from the original master pattern. The mould can be male or or female depending on the specific requirements of the article to be moulded, the important factor being that the surface of the moulding which comes in contact with the mould will be smooth, giving not only a better appearance, but improved resistance to weathering or chemical attack. A moulded outer surface, as obtained with a female mould is clearly preferable for articles such as boat hulls or motor car bodies, whereas a smooth interior surface as provided by a male mould is required for storage tanks or pipes. A female mould is preferred for the majority of general purpose mouldings.
Making the pattern
The pattern is essentially a reproduction of the article which is to be manufactured and can indeed be an example of the article itself made from another mould. It is usually made individually from more easily workable materials which would not withstand the repeated usage required of the mould. When designing the pattern, it is important to remember that a shape allowing easy removal of the mould will in turn give a mould permitting easy removal of finished mouldings. For this reason, a taper of 1° - 11/f is desirable on vertical faces and undercuts or sharp corners should be avoided whenever possible.
The materials commonly used in the construction of patterns include-wood, sheet metal and plaster of Paris. Before a mould can be made from the pattern, the surface must be sealed to prevent absorption of polyester resin by any porous materials. This is usually achieved by application of seven or eight layers of a sealer and, when this is dry, sanding and wax polishing the surface to give a smooth, glossy finish.
Making the Mould
Mould release agent No. 3 is first applied to the pattern. It is important to avoid streaks or bare patches since these will show as marks on the surface of the mould and application of the release agent by spraying or by means of a sponge, rather than with a brush, will probably give the best results. RELEASE AGENT MUST BE ALLOWED TO DRY.
The next stage in preparing the mould from the pattern involves the application of a layer of gel coat resin. This will give a surface on the mould free from glass reinforcement and reproducing the glossy finish of the pattern. It is often desirable to colour this gel coat so that when the final mould is in use, it will be easy to see if thin areas are present as gel coat is subsequently applied when making a moulding. Catalyst is added to the gel coat resin at the rate of 2% (20 cc per kilo of resin) which is then brushed on under dry conditions and preferably at a temperature of 18°C - 20°C.
About 600 - 700 grm/m2 will give a suitable thickness of gel coat.
When the gel coat has cured sufficiently, usually after 2-2]/2 hours, a thin coat of general purpose lay-up resin is applied to the mould followed by a glass surfacing tissue, ensuring that there are no voids between this tissue and the gel coat. The resin is allowed to gel and the first layer of chopped strand mat (300g/m2 or 1oz/ft2 is then applied together with sufficient lay-up resin to give a resin/glass ratio of 2 : 1 and 2!^ : 1, any trapped air being removed by use of a paddle roller. Where two faces meet, the glass will tend to bridge the corner and should be cut and joined to ensure that this does not occur. When impregnation has been completed satisfactorily and all air removed, the resin is left to gel and cure overnight.
On the second day, a layer of thicker glass mat (450g/nf or Ilkiz/ft2) and resin is applied with similar precautions and again allowed to cure overnight, followed by a further layer on the following day. When this has cured, the mould can be built up to the required thickness with the bulk of the resin and reinforcement, the amount used depending on the intended life of the mould. To ensure that the mould will not distort during removal from the pattern or in subsequent use, stiffen-ers are incorporated where necessary. These usually consist of shaped pieces of wood, the areas which they are to cover being previously wetted with resin and covered with a layer of glass reinforcement. When the stiffeners are in position, the resin retaining them is allowed to cure overnight and next day they are completely covered by a layer of resin and glass mat, so as to completely enclose the wood, thus preventing any change in its moisture content which could otherwise cause warping. As soon as this resin has cured, the construction of the mould is complete, but, for the best results it should be allowed to remain on the pattern for another three or four days. Care should be taken in removing the mould from the pattern. Wedges of a fairly soft material such as wood can be used and when removal is difficult, it is helpful to introduce water between the mould and the pattern to dissolve away the release agent. When released from the pattern, the mould should be carefully inspected for any minor blemishes and repaired where necessary before being put into service.